SUMMARY?COREC Thereisextensiveunexplainedgeographicandorganizationalvariationinhealthcareutilizationandoutcomes acrosstheUnitedStates.Manyhealthpolicyexpertsviewthereductionofunwarrantedvariationtobe essentialinimprovinghealthcarequalityandreducingcosts.Previously,individualheterogeneityofclinicians, hospitals,andhealthsystemshasbeenstudiedasapossibledeterminantofsuchvariation.However, substantialvariationisleftunexplained.Wehypothesizethatthebestwaytomakefurtherinroadsisinthe contextofcliniciannetworksandthatthearrayofprofessionalrelationshipsbetweencliniciansacrosstheU.S. isthemechanismforthediffusionanddisseminationofbest-practice(highvalue,lowcost)andworst-practice (ineffectiveandexpensive)care.Unfortunately,ithasprovenchallengingtomeasureprofessionalnetworks amongU.S.providers.TheMethodsCorewillmeetthischallengeby(i)measuringrelationshipsfrompatient- clinicianencountersinMedicareclaimsdatausingmethodsthatmakegreateruseoftheinformationcontained inthemthanpreviously,(ii)computingnetworkmeasuresandstatisticalmodelsinvolvingthemfortheentire U.S.socialnetwork,and(iii)usingadvancedcomputingtoimplementcomputationsontheentirenetworkin feasibletime.Advancesinstatisticalmethodsformodelinghierarchicallystructureddataandcausally- defendablecomparativeeffectivenessresearchwillalsobemade.Thesemethodologicalinnovationssupport andenhancethefiveprojects,providingthemthebasistodelvedeeperintotheirvariouslinesofenquirythan wouldbepossibleotherwise.